Aneurysms arise when a thinning, weakening section of an artery wall balloons out and are often treated when the aneurysm diameter is more than 150% of the artery's normal diameter. Aneurysms are estimated to cause approximately 32,000 deaths each year in the United States. Additionally, aneurysm deaths are suspected of being underreported because sudden unexplained deaths, about 450,000 in the United States alone, are often simply misdiagnosed as heart attacks or strokes while many of them may be due to aneurysms.
The most common and deadly form of aneurysms occur in the aorta, the large blood vessel stretching from the heart to the lower abdomen. Aortic aneurysms are detected by standard ultrasound, computerized tomography (CT) and magnetic resonance imaging (MRI) scans, and the increased use of these scanning techniques for other diseases has produced an estimated 200% increase in the diagnosis of intact aortic aneurysms. A normal aorta is between 1.6 to 2.8 centimeters wide; if an area reaches as wide as 5.5 centimeters, the risk of rupture increases such that surgery is recommended.
U.S. surgeons treat approximately 50,000 abdominal aortic aneurysms each year, typically by replacing and/or bypassing the abnormal section of vessel with a plastic or fabric graft in an open surgical procedure. A less-invasive procedure that has more recently been used is the placement of a stent graft across the aneurysm site. Stent grafts are tubular devices that span the aneurysm to provide support without replacing a section of the vessel. The stent graft, when placed within the artery at the aneurysm site, acts as a barrier between blood flow and the weakened wall of the artery, thereby decreasing pressure on the damaged portion of the artery. This less invasive approach to treat aneurysms decreases the morbidity seen with conventional aneurysm repair. Additionally, patients whose multiple medical comorbidities make them excessively high risk for conventional aneurysm repair are candidates for stent grafting. Stent grafts have also emerged as a new treatment for a related condition, acute blunt aortic injury, where trauma causes damage to the artery.
While stent grafting represents an improvement over previously-used vessel replacement techniques, there are still risks associated with the procedure. One of these risks is migration of the stent graft due to hemodynamic forces within the artery. Graft migrations lead to endoleaks, a leaking of blood into the aneurysm sac between the outer surface of the graft and the inner lumen of the blood vessel. Graft migration and resulting endoleaks are especially possible in curved portions of vessels where asymmetrical hemodynamic forces in the area can place uneven forces on the stent graft. Additionally, the asymmetrical hemodynamic forces can cause remodeling of the aneurysm sac which leads to increased risk of aneurysm rupture and increased endoleaks.
Based on the foregoing, one goal of treating aneurysms is to provide a stent graft that does not migrate. In an attempt to achieve this goal, stent grafts with stainless steel anchoring barbs that engage the vessel wall have been developed. Additionally, endostaples that fix the graft more readily to the vessel wall have been developed. While these physical anchoring devices have proven to be effective in some patients, they have not sufficiently ameliorated the graft migration and endoleak problems associated with current stent-grafting methods and devices in all cases.
An additional way to reduce the risk of stent graft migration is to administer to the treatment site, either before, during or relatively soon after stent graft implantation a cell growth promoting factor. This administration can be beneficial because, normally, the endothelial cells that make up the portion of the vessel to be treated are quiescent at the time of stent graft implantation and do not multiply. As a result, the stent graft rests against a quiescent endothelial cell layer. If cell growth promoting compositions are administered immediately before, during or relatively soon after stent graft deployment, the normally quiescent endothelial cells lining the vessel wall, and in intimate contact with the stent graft, will be stimulated to proliferate. The same will occur with smooth muscle cells and fibroblasts found within the vessel wall. As these cells proliferate they can grow into and around the stent graft lining such that portions of the stent graft becomes part of the vessel lumen rather than merely pressing against its surface. This endothelialization helps to prevent stent graft migration. In addition to cell growth promoting factors, it can be beneficial to administer a number of other bioactive materials, such as, without limitation, anti-inflammatory agents and/or anti-coagulant compounds.
While it can be beneficial to administer a bioactive material in an area to be implanted with a stent graft. This administration can also be problematic, however, because an increase in volume and internal pressure near an aneurysm site caused by the administration of an exogenous substance can increase the likelihood of aneurysm rupture. Therefore, a need exists for methods and devices that can administer a bioactive material to an aneurysm site while maintaining nearly-constant pressure in the area. Such methods and devices would allow for the introduction of beneficial bioactive materials without increasing the likelihood of aneurysm rupture.